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The study was conducted to establish the type and severity of
Trypanasoma congolense infection in donkeys, particularly the clinical picture,
and to correlate this with the effects on draught power and the role these
animals play in the perpetuation of the disease. Five mature donkeys were
subcutaneously inoculated with 7.5 x 106 blood stream forms of T. congolense,
strain IL 3575. Three donkeys in adjacent fly proof stalls were kept as controls.
The donkeys were monitored clinically on daily basis for a period of three
months. Blood was collected from the jugular vein twice a week for
haematology and once a fortnight for serum biochemistry. Blood from the
marginal ear vein was drawn into heparinized capillary tubes for diagnostic
purpose. Thin buffy coat smears were made, stained in 1:5 Giemsa for 15 - 20
minutes and observed under oil-immersion for the presence or absence of
trypanosomes. Subinoculations into mice and sheep were done to verify whether
the trypanosomes observed in donkeys were viable and patho~enic. The degree
of anaemia in sheep and the rapidity at which mice died was the measure of
pathogenicity. Donkeys were euthanized after three months and thorough post
mortem examinations done. Samples were collected from the lymphoid organs,
heart, lungs, skeletal muscles, kidneys, gastrointestinal-tract, brain and glandular
organs whether showing gross lesions or not. They were fixed in ten per cent
formalin, embedded in paraffin wax, sectioned at thickness of 6pm and stained
with haematoxylin and eosin ( H & E).
The infection had a prepatent period of 29 - 41 days in donkeys. The
clinical picture indicated a subclinical infection with no signs but a positive
diagnosis was made using thin stained buffy coat smears. Pyrexia peaks
accompanying parasitaemic phases in ruminants and horses were lacking. The
respiratory and heart rates were elevated during the third month of infection
when pounding heart beats were picked on auscultation, and the mucus
membranes were pale. Attempts to quantify the parasites in blood using a
haemocytometer were unsuccessful because they were low in numbers and
appeared sporadically. These trypanosomes were morphologically similar to
those inoculated into the donkeys.
The results showed red blood cells to decrease by 46.7% ; the packed
cell volume by 41.6% haemoglobin concentration by 41.4% in the infected
donkeys. In the control group, the red blood cells decreased by 28.6%; the
packed cell volume by 22.2% and the haemoglobin concentration by 26.8%.
This indicated that a decrease of 18.1%, 19.4% and 14.6% in the red blood cell
count, packed cell volume and haemoglobin concentration, respectively in
infected animals was attributed to T. congolense infection. Mean corpuscular
volume, mean corpuscular haemoglobin and mean corpuscular haemoglobin
concentration increased over pre-infection values by 15.8%, 30.1% and 6.3%,
respectively in infected animals. Thus the anaemia observed could
morphologically be classified as normochromic, macrocytic. The leukocytes
counts showed minimal changes although a relative decrease in neutrophils and
a relative increase in lymphocytes was observed in infected animals. Monocytes
counts were higher in infected animals. This depicts an efficient regulatory
mechanism(s). The fall in total plasma proteins plus a rise in eosinophil count
in both groups of animals suggested a helminth infection. This was confirmed
at post-mortem when helminth parasites were observed in the gastro-intestinal
tract from the stomach, intestines to the colon. The parasites were mainly the
small and large strongyles. This concurrent helminthiasis contributed to the
decrease of 22.2% in packed cell volume in the two groups. The total bilirubin
rose to 0.8mg% from 0.2 - 0.5mg% while blood urea nitrogen increased to
89mg% from 12 - 40mg%. These values were all within the ranges provided
by other workers.
At necropsy, the infected donkey carcasses were in fair body condition
but moderate amounts of straw - coloured fluid was found in all the body
cavities. The liver was slightly enlarged while the spleen displayed prominent
Malphigian corpuscles. Microscopically, the lymph nodes had follicular
hyperplasia characterized by many prominent follicles. Medullary cords were
enlarged and had increased number of plasma cells as well as macrophages.
The spleen had follicular hyperplasia of the Malphigian corpuscles, the
peri arteriolar lymphatic sheaths were expanded while the marginal zones closely
resembled the white pulp. Macrophages and plasma cells were encountered in
the red pulp while mitotic figures of medium-sized lymphocytes were
occasionally observed in the follicles. The lungs had patches of atelectasis,
kidneys displayed areas of interstitial nephritis while the liver showed hydropic
degeneration and occasional helminth larval migratory tracts.
The mice peaked in parasitaemia 10 - 15 days after inoculation. Most
of these died within 30 days although a few of them survived for more than 60
days. In sheep, a parasitaemia appeared 7 - 17 days after inoculation and
persisted in transient peaks to the end of the experiment. A decrease of 17.5%
in packed cell volume was attributed to trypanosomiasis. These observations
suggests that donkeys may be potential reservoirs of T. congolense infections
in endemic areas.